The present study is designed to obtain information about biochemical events in virus-induced leukemogenesis in vivo; that is, within the environment that promotes or defends against the developing neoplasm. The focus is on early events which may hopefully provide a focal point for interfering with the malignant evolution. Goals for the coming year include (a) to assess the potential importance of a phosphodiesterase activity that appears to be ectopic in nature and that changes dramatically during leukemogenesis; (b) to determine whether enzyme shifts during leukemogenesis may result from the selective expansion of cell types present in the normal thymus. Biochemical parameters of cells separated on the basis of size and density from normal and malignant thymuses will be measured; and (c) to continue preliminary experiments concerning the usefulness of the Gross-virus injected C3H mouse as a model. Bibliographic references: In press, Kemp, R.G., Hsu, R.-Y., and Duquesnoy, R.J., Changes in Lymphoid Cyclic Adenosine 3':5'- Monophosphate Metabolism During Murine Leukemogenesis, Cancer Res, 1975.